In recent years, in semiconductor devices for power control of electric trains, hybrid vehicles, and electric vehicles, operation temperatures thereof have risen due to use of wide gap semiconductors. In silicon carbide (SiC) semiconductors and the like which have been particularly focused upon, since the operation temperature is 200° C. or higher, a packaging material therefor needs to have high heat resistance of 250° C. or higher. In addition, due to the rise in the operation temperature, thermal distortion may occur due to a difference between thermal expansion coefficients of materials used in a package, and there is also a problem of a reduced lifespan due to peeling off of a wiring or the like.
As a method of solving such a heat resistance problem, highly thermally conductive ceramic substrates such as aluminum nitride and silicon nitride and highly heat resistant organic resins and silicone resins combined with inorganic fillers for improving thermal conductivity have been developed and particularly the development of high heat resistance resins such as oxazine resins and high heat resistance silicone resins have progressed. In Patent Literature 1, a polybenzoxazine-modified bismaleimide resin having excellent heat resistance is disclosed. However, compounds that exhibit sufficient heat resistance and durability have not yet been utilized, and thus the development of materials with higher heat resistance has been performed.
As another method of solving a heat resistance problem of a member, there is a method in which thermal conductivity is improved, unevenness in temperature is reduced, and as a result, localized high temperatures are reduced. In addition, it can also be expected that there will be an effect in which, when the thermal conductivity is high, a temperature of a part in contact with something will be unlikely to rise. Generally, the introduction of many cyclic structures into a main chain of molecules in order to increase thermal conductivity of a resin component has been examined. In addition, it is known that high linearity of molecular chains is preferred in order to improve thermal conductivity of such resins. Examples of a compound having many cyclic structures and linearity include a liquid crystal compound.
In Patent Literature 2, as a method of improving the thermal conductivity of a resin, a method in which a liquid crystal composition containing a liquid crystal compound having a polymerization group at both ends is alignment-controlled using an alignment control additive, a rubbing treatment method, or the like, polymerization is performed in a state in which the alignment state is maintained, and thus a resin film having high thermal conductivity is obtained is disclosed.